The present invention relates to the technical field of adhesive compositions based on renewable raw materials.
The present invention relates more particularly to adhesive compositions, especially pressure-sensitive adhesive compositions, based on renewable raw materials, and also to their use.
The present invention further relates to a method of joining at least two substrates by adhesive bonding.
The concept of “adhesive bonding” describes in general a manufacturing process for the physical joining of substrates. With adhesive bonding, the adhesive adheres by physical interaction—occasionally also by chemical interaction—referred to as adhesion, to the substrates, and connects them usually permanently. Since adhesive bonding on the one hand permits extensive and force-mediated joining of the adherends and on the other hand, on the basis of its non-aggressive properties, is suitable for connecting almost all materials to one another, adhesive bonding techniques are employed diversely both for domestic requirements and for industrial application. Other joining techniques, such as welding or soldering, for example, but also screwing, are increasingly being replaced by adhesive bonding methods. Around 50% of the goods produced in Germany, for instance, are now associated with adhesives.
Of increasing importance in this context are pressure-sensitive adhesives (PSAs), which are non-curing, permanently tacky adhesives whose chemical state exhibits little or no change after the adhesive has been completed. PSAs possess a permanent surface tack, as it is known, which enables them to bond by gentle applied pressing, without supply of thermal energy and without chemical reactions. PSAs typically possess a dynamic build-up of adhesion; in other words, the ultimate strength of the adhesive bond is achieved only after a number of minutes or even days. On the basis of their specific mode of application, PSAs are divided into three groups:
One widespread possibility for use of PSAs is in restickable products, for which the adhesive has only a low bond strength. The adhesive system is undamaged on detachment and is capable of repeated bonding. Examples of this are sticky notes or closure bonds, such as those of paper handkerchief packs or of wet-wipe packs, for example.
Another widespread possibility for use of PSAs is in releasable products in which the PSA is of moderate to high bond strength. With these products, the adhesive bond, designed for permanent bonding, can be released again, often, indeed, with the possibility of residue-free release of the adhesive bond. Releasable products in this sense are, for example, plasters, stickers, labels, self-adhesive sheets and packaging tapes.
Lastly, PSAs are also being employed increasingly for permanent-bonding products, the PSA in this case possessing a particularly high bond strength and being suitable for semi-structural applications. Permanent-bonding products with PSAs are therefore often used for industrial applications, such as in the fixing of exterior mirrors or protective strips or on damping-stiffening elements in vehicle construction, for example. Other applications are in window and facade construction, in mirror bonds, and also in the production of electronic articles, such as mobile phones, for example, or else of self-adhesive products which on detachment are deliberately to be visibly altered or destroyed, such as model plates or testing-certificate roundels for cars.
Adhesive systems in general, and including PSA systems in particular, are based primarily, if not, indeed, exclusively, on petrochemical products. A consequence of this is that, given growing demand for crude oil and crude oil-based products in tandem with limited supply, the price of crude oil and hence also of crude oil-based products is continually rising. Accordingly, adhesives and the products on which they are used are at the same time becoming more costly to produce, which means that the use of adhesives is in certain products no longer profitable or is making those products more expensive.
Furthermore, the use of crude oil-based products, more particularly adhesives, contributes to further consumption of fossil raw materials, meaning that their use leads to an unfavourable CO2 balance and, ultimately, contributes to global warming and to climate change.
Against this background there has been no lack of attempts in the prior art to provide adhesive systems based on renewable raw materials.
For instance, US 2001/0045604 A1 discloses a multi-layer biodegradable film which has a layer of a biodegradable PSA. The acrylate-containing biodegradable PSA actually employed, however, is a petrochemical product and not, for instance, a product based on renewable raw materials.
US 2008/0057300 A1 describes biodegradable two-component fibres which are based on polylactides and which develop tack on heating. The fibres described therein do not possess hot-melt PSA suitability.
Furthermore, DE 1 99 44 159 A1 relates to water-soluble, low-emission and biodegradable adhesive mixtures based on polyaspartic acid and/or the salts and derivatives thereof. The adhesive mixtures consist solely of polyaspartic acid and/or its salts and/or derivatives, and also natural adhesive resins. This specific combination of ingredients is suitable only for a narrowly confined field of application.
EP 0 741 177 A2 describes hot-melt adhesive compositions which are prepared from a thermoplastic polymer which is biodegradable or can be decomposed by water, with the assistance of sucrose benzoate as tackifier.
Furthermore, EP 0 899 274 A1 relates to graft copolymers for which polylactide units are applied by means of graft polymerization to a saturated polymeric backbone, and which can be used as a thermoplastic elastomeric component in a hot-melt adhesive.
WO 96/05264 A1 discloses PSAs based on poly(β-hydroxyorganyls).
EP 0 438 426 B1 relates to a system for the controlled delivery of bioactive constituents, the constituents being held by a polymeric composition comprising sections of poly(R-lactide) interlocked with sections of poly(S-lactide).
DE 100 54 277 A1 relates to tearing strips whose parent polymeric films and also coatings applied thereto are biodegradable. The tearing strips can be used in conjunction with biodegradable packaging materials.
Furthermore, DE 692 03 713 T3 discloses a hot-melt adhesive based on polyhydroxybutyrate/hydroxyvalerate.
Furthermore, WO 94/10257 A1 discloses a hot-melt adhesive composition based on polylactides.
DE 198 55 100 A1 discloses an adhesive which is biodegradable.
Furthermore, DE 694 22 053 T2 describes biodegradable or compostable hot-melt adhesives which comprise polyesters based on lactic acid.
Moreover, WO 02/070583 A2 describes biodegradable polymeric compounds based on polylactides and also their use as hot-melt adhesives.
With the aforementioned polymers and adhesive systems, however, it is not possible to provide adhesive systems, more particularly PSA systems, which have capacity to compete, both economically and on a performance basis, with petrochemically based adhesive systems.
More recently, however, there has been increased demand for biobased and/or biodegradable adhesives. Drivers for the elevated demand include on the one hand an ever-growing awareness of environmental issues, and on the other hand the more and more frequent supply bottlenecks and associated price rises in crude oil-based raw materials such as, for example, styrene-isoprene-styrene block copolymers and hydrocarbon resins, which constitute the main constituents of the PSAs presently produced.
In the market for food packaging, for example, there is interest in biodegradable products. Within the sector of plastics for producing films or trays for foods, or else for disposable tableware, there is already a relatively wide selection of biobased and biodegradable products. Base materials employed here are, for example, alongside PLA, PHB and modified starch. Through the use of adhesives based on renewable raw materials it would be possible to provide entirely biodegradable, biobased packaging.
The use of renewable raw materials in adhesives applications often multiplies the production costs, meaning that, in spite of the improved CO2 balance and the improved environmental effect, the use of these adhesive systems in industrial applications does not make economic sense. Moreover, adhesive systems based on renewable raw materials often fail to attain the same properties as their petrochemical-based counterparts. This is true especially of the bond strength and also of the resistance of the bond towards environmental effects, such as temperature changes, for example, or else chemicals.
As a result of this, there is currently on the market no adhesive system available based on renewable raw materials that would be able in terms of its applications properties and its profile of properties to compete with petrochemical-based adhesive systems, let alone one which would be economically competitive. In particular there has to date been no hot-melt PSA available that is based on renewable raw materials.